FIG. 1 illustrates a conventional GaAs-based vertical-cavity surface-emitting laser (VCL) 10 as known in the prior art. The epitaxial design allows the basic VCL Bragg-stack mirrors 12 & 14 and active gain layers to be grown during a single closely-controlled crystal growth process. The active region 24 contains one or more quantum wells (QW) with a typical thickness of .about.8 nm placed between .about.10 nm barrier layers. The active region 24 is surrounded by spacer layers 16 to provide optimal position of the active layer within the resonant cavity. Electrical VCL excitation occurs by injecting current through the active (QW) region causing electron-hole recombination and subsequent photon emission. Various methods for current confinement within the active region include proton implantation, etched mesa, dielectric aperturing, or buried heterostructure designs [Margalit et al. 1997; Jewell et al. 1991; Gourley et al. 1995]. Typical laser diameters range from a few microns for single mode lasers to 50 .mu.m or larger for multi-mode lasers. Electrical contacts can be implemented, for example, by forming heavily doped n- and p-type layers 18 & 19 on the top and bottom surfaces followed by deposition of thin metal layers (Au, Al). Alternately, contact layers can be formed using optically transparent conducting materials such as indium-tin-oxide (ITO) [Chua et al. 1997].
Conventional current injection VCLs utilize both n-type and p-type multilayer Bragg-stack mirrors formed by pairs of alternating high and low refractive index layers. Each typical semiconductor Bragg-stack mirror requires anywhere between 30 and 100 layers depending on the specific semiconductor material system used to achieve the high reflectivity required for laser operation. One disadvantage of the conventional Bragg-stack mirror configuration is that between 60 to over 200 high quality layers may be required to fabricate a complete VCL.
In a VCL, it is established physics that the small thickness of the gain regions limits the amplification of the oscillating light intensity obtained with each pass through the active region. In contrast, for in-plane or edge emitting lasers, the gain length can be hundreds of micrometers. For initiation of lasing oscillation, the round trip gain must equal round trip loss. Therefore, the mirror transmission loss in a conventional VCL must be exceedingly small, requiring highly reflective dielectric mirrors with low material absorptive losses. Typically, for conventional VCLs, this reflectivity must exceed 99%, with values approaching 99.9% often used. As an example, for light output at wavelength .lambda.=850 or 980 nm with AlAs/GaAs Bragg mirrors (layer refractive-index difference .DELTA.n=0.57), .about.30 periods (60 layers) are required to achieve a 99.5% reflectivity [Margalit et al. 1997].
Despite the rigid requirements outlined above, VCLs are promising sources for optical communications if these problems can be overcome. In particular, the laser output of a VCL matches the circular aperture of optical communications fibers. Moreover, VCLs can be more cost effective and have better performance than in-plane and edge-emitting lasers.
Research work has previously been done on diffractive coupling and on guided-mode resonance (GMR) filters. For example, applying exact electromagnetic analysis to optical multilayer structures containing diffractive and waveguide layers, sharp waveguide-coupling resonance phenomena were found and their origins were explained [Magnusson et al. 1992]. Combining resonant waveguide gratings with thin-film antireflection layers yields reflection filters (i.e., the useful spectrum reflected as in a mirror) with high efficiency (.about.100%), arbitrarily narrow passbands, polarization selectivity, symmetric line shapes, and extended low sidebands [Wang et al 1994]. Thus, reflection GMR filters with efficiencies exceeding 90% have been fabricated.
U.S. Pat. No. 5,216,680 by Magnusson et al. discloses optical GMR filters and suggests that they can be placed within a laser. However, this patent does not address VCLs and does not contemplate or suggest any such structure.
U.S. Pat. No. 5,598,300 by Magnusson et al. discloses ideal or near ideal filters for reflecting or transmitting electromagnetic waves using GMR effects. This patent provides detailed teachings of various structures of GMR filters and how to design these filters for desired performance characteristics. The patent suggests that these filters could be used as mirrors and phase-locking elements for VCLs. However, the patent does not provide any teaching or suggestion of how to implement such an application.
The present invention addresses the problems and shortcomings in the prior art by providing more efficient VCLs that are simple to manufacture, that have high gain with reduced threshold current and reduced number of layers, and that allow for lower mirror reflectivity.